1. Field of the Invention
The present invention relates to a fuel injection device of an internal combustion engine capable of varying the time number of fuel injection(s) performed by an injector according to an operation state of the internal combustion engine.
2. Description of Related Art
A fuel injection device of an internal combustion engine capable of varying the time number of fuel injection(s) according to an operation state of the internal combustion engine is described in Patent document 1 (JP-A-H7-119507), for example.
The fuel injection device of Patent document 1 is applied to a direct-injection engine that injects the fuel directly into a combustion chamber. The fuel injection device starts the fuel injection from a latter half of a compression stroke to perform stratified charge combustion when an engine load is low. The fuel injection device starts the fuel injection from a former half of an intake stroke to perform homogenous combustion when the engine load is high.
In the stratified charge combustion performed when the engine load is low, the fuel is injected only once from the latter half of the compression stroke, whereby a mixture gas having a concentration providing suitable ignitability is formed near an ignition plug. When an operation state of the engine is in a low-rotation operation range where the homogenous combustion is performed, i.e., when the operation state is a high-load low-speed operation state, the fuel is injected via multiple times of divided injections, which are performed from the former half of the intake stroke. Accordingly, the fuel injection quantity injected per each injection decreases. As a result, the fuel is atomized and diffused effectively and a homogeneous mixture gas is formed in the combustion chamber.
The fuel injection device usually has a booster circuit for generating a high voltage from a battery voltage to drive the injector. The generated high voltage is temporarily stored in a charge capacitor. The high voltage is applied to the injector through a switching element from fuel injection start timing to fuel injection stop timing. Thus, a quantity of the fuel corresponding to the applying time of the high voltage is injected from the injector.
The injector has a delay since the high voltage is applied to the injector until an injection hole actually opens and the injection is started. The delay is an invalid injection time, in which the fuel is not injected from the injector although the high voltage is applied to the injector.
If a driving time of the injector equivalent to a necessary fuel injection quantity is simply divided by the injection time number and the injector is driven as in the above-described fuel injection device, accuracy of the fuel injection quantity deteriorates because the number of occurrence(s) of the invalid injection time differs between the one-time injection and the two or more times of injections. As a countermeasure, correction can be performed to lengthen the driving time of the injector by a predetermined invalid injection time in accordance with the injection time number.
However, if the injection by the injector is divided into two or more times of the injections, the divided injections are performed at extremely short time intervals. Therefore, the injector is driven while continuously consuming the high voltage stored in the charge capacitor. That is, the injector is driven by the high voltage stored in the charge capacitor in the first injection. However, in the second or later injection, the voltage has been consumed by the preceding injection(s), so the injector is driven by the voltage having lowered from the initial high voltage.
The invalid injection time mentioned above changes with the level of the voltage applied to the injector. Therefore, length of the invalid injection time differs between the first injection and the second or later injection. The invalid injection time of the second or later injection is longer than the invalid injection time of the first injection. Moreover, it is thought that the invalid injection time of each injection time number changes with time due to degradation of the charge capacitor and the like. Therefore, even if the driving time of the injector is corrected by the predetermined invalid injection time according to the injection time number, it is difficult to sufficiently improve the accuracy of the fuel injection quantity.